Sounds a bit too risky for my blood. I will just sell this PSU and pick up one that has some more room to breathe.
I have 2 x 7950 running in a rig on a killawatt
its showing ~550W pulled from the wall
a 650 *should* be fine
I have a 1kw on it though because its what I had
and I couldnt shoehorn the 6870 I have into due to mounting issues